1. Field of the Invention
The invention relates to a method for manufacturing a rack housing and the rack housing manufactured by the method.
2. Description of Related Art
A rack-and-pinion steering system in an automobile and the like uses a rack housing to fix a rack shaft to a vehicle body while supporting the rack shaft so as to allow free linear reciprocating motions of the rack shaft. The rack housing is formed to be elongate in an axial direction of the rack shaft and receives a steering reaction force transmitted from wheels at the time of steering. Thus, the rack housing as a whole is generally formed of an aluminum die cast material or the like with excellent strength and rigidity.
However, recent demands for energy saving has led to a demand for a reduction in the weights of automotive components. Thus, also for the rack housing, the weight of which occupies a large portion of the total weight of the steering system, effort is being made to achieve a weight reduction while ensuring the needed strength and rigidity. For the weight reduction, a cylindrical portion of the rack housing through which the rack shaft is inserted may be formed by molding a prepreg including textile fibers impregnated with a thermosetting resin into a tubular shape as described in Japanese Patent Application Publication No. 2013-208927 (JP 2013-208927 A).
Japanese Patent No. 5136876 (JP 5136876 B) describes a prepreg including two layers of a reinforced fiber base material secured together with a binding agent of a thermoplastic resin placed between the layers. Japanese Patent Application Publication No. H7-140262 (JP H7-140262 A) describes, as a prepreg, a plastic case formed of a carbon fiber fabric impregnated with a resin.
For the prepreg of the thermosetting resin as described in JP 2013-208927 A, a long time is needed to form the cylindrical portion of the rack housing. That is, the formation takes approximately five hours or more from the start of heating until the cylindrical portion is completed by completely curing the thermosetting resin, including a fluidization time needed to heat and soften a sheet-like prepreg in order to mold the prepreg into a tubular shape and a temperature increase time needed to raise the temperature of the prepreg to the curing temperature of the thermosetting resin after the molding of the prepreg into the tubular shape.
Given that a cycle time desired for manufacture of automotive components is approximately one minute, the above-described time is very long and much energy is consumed by the continuous heating during the time, leading to concern for increased costs. The use of a prepreg of a thermoplastic resin as described in JP 5136876 B enables a reduction in the cycle time for the rack housing. However, the prepreg is a thick sheet that is difficult to deflect. Thus, when the prepreg is pressed at once to form the rack housing, defects such as delamination and wrinkling may occur in the prepreg.